The present invention generally relates to metal halide lamps, and more specifically relates to a metal halide lamp that relies on the application of a high voltage to start the lamp and that uses a starting aid to reduce the starting voltage of the lamp.
Metal halide lamps start upon application of a high voltage between two main electrodes or to an inductive start system. Metal halide lamps which do not contain UV enhancers require higher voltage pulses to release avalanche initiating electrons. Initiating electrons, in this manner, are believed to be released from the electrode by field emission or by field extraction from charges in shallow traps on the wall of the arc tube. However, not all sockets into which such lamps are inserted have the capacity to carry the high voltage needed to start the lamps. Accordingly, a starting aid, also known as an ultraviolet (UV) enhancer, is provided in such lamps. The UV enhancer emits UV radiation that causes the release of photoelectrons into the main body of the lamp. The photoelectrons reduce the voltage needed to start the lamp. Rapid starting eliminates the sockets from being stressed by long-term exposure to the high starting voltages. This reduces the probability of socket failure.
The UV enhancer also reduces the statistical lag time between the time of application of the high voltage and the lamp breakdown (ignition) as defined by the drawing current. This is important in mercury-free lamps because such lamps typically have a ballast with a time-out feature. The ballast attempts to start the lamp for a predetermined period of time and then shuts off. If the statistical time lag is too long, the ballast interprets the delay as an inoperative lamp and shuts off too soon.
A typical metal halide lamp includes a discharge vessel in an outer bulb. The discharge vessel has two electrodes that receive the voltage for starting the lamp. The UV enhancer is located within the outer bulb and connected to one of the electrodes. The UV enhancer is positioned close to the other electrode to allow capacitive coupling. A gas inside the UV enhancer is partially ionized by the capacitive coupling and emits UV light that aids in starting the lamp. Construction and operation of such lamps is well known and described, for example, in U.S. Pat. No. 5,942,840 that is incorporated by reference. The lamp may also be electrodeless, such as described in U.S. Pat. No. 5,070,277 that is also incorporated by reference.
The conventional UV enhancer is a capsule with a sealed cavity that contains a gas or a mixture of gases, such as mercury vapor and an inert gas (argon, helium, krypton, neon, or xenon). An electrode extends into the cavity and provides a voltage from one of the discharge vessel electrodes. Upon application of the starting voltage, a capacitive discharge starts in the capsule causing the capsule to emit UV radiation, which in turn causes the release of photoelectrons in the lamp, which in turn lowers the voltage needed to start the lamp.
The practical and legal reasons for avoiding the use of mercury in lamps are well known. While much attention has been directed to removing mercury from the main lamp (e.g., the discharge vessel), the starting aid still may contain mercury. The effort to remove mercury entirely from lamps has included removing the mercury vapor from the UV enhancer so that the sealed cavity includes only an inert gas, typically argon. However, insufficient UV radiation escapes the capsule when argon is used and this solution is not satisfactory for most lamps.
The invention is an improvement in which the starting aid does not include mercury, thereby allowing the metal halide lamp to be entirely mercury-free. The starting aid of the present invention uses iodine and an inert gas instead of mercury.
An object of the present invention is to provide a novel UV enhancer that avoids the problems of the prior art and provides sufficient UV radiation by employing iodine and an inert gas instead of mercury.
A further object of the present invention is to provide a novel UV enhancer for a metal halide lamp that includes a UV-transmissive capsule with a cavity in which iodine and an inert gas are sealed, wherein the iodine emits UV radiation when excited to reduce a starting voltage of the lamp.
Another object of the present invention is to provide a novel metal halide lamp that includes a discharge vessel inside an outer tube, and a UV transmitting starting aid in the outer tube that includes a capsule with a cavity that has iodine and an inert gas sealed therein.
Yet another object of the present invention is to provide a novel method of starting a metal halide lamp in which a starting voltage of the lamp is lowered by exciting iodine sealed with an inert gas in a UV enhancer to cause emission of UV radiation that lowers the starting voltage of the lamp.